Various processes can be used to produce polyethylene, including gas phase processes, solution processes, and slurry processes. In ethylene slurry polymerization processes, diluents such as hexane or isobutane may be used to dissolve the ethylene monomer, comonomers and hydrogen, and the monomer(s) are polymerized with a catalyst. Following polymerization, the polymer product formed is present as a slurry of polyethylene particles suspended in the liquid medium.
In typical multi-reactor cascade processes, shown e.g., in WO 2005/077992 A1 and WO 2012/028591 A1, the reactors can be operated in parallel or in series, and the types and amounts of monomer and conditions can be varied in each reactor to produce a variety of polyethylene materials, including unimodal or multimodal polyethylene material. Such multimodal compositions are used in a variety of applications; e.g., WO 2012/069400 A1 discloses trimodal polyethylene compositions for blow moldings.
A potential challenge encountered using continuous stirred tank reactors in ethylene slurry polymerization systems is the fouling that can occur on the reactor internals. For instance, ethylene monomer is introduced into the reactor in gaseous form and dissolves in the diluent. The solid catalyst component is dosed into the reactor and is suspended in the diluent. When the dissolved ethylene comes into contact with the catalyst particles, polyethylene is formed. The reaction occurs throughout the reactor, including near the interior reactor surfaces and reactor internals, and the area around the ethylene inlet nozzles since the local concentration of ethylene is at its highest at the discharge of the inlet nozzle. The ethylene feed, in many such reactions, would immediately dissolve and be mixed so as to form a uniform concentration in the diluent in contact with uniformly distributed catalyst particles. However, if dissolution of the ethylene and mixing of the reactor contents is not adequate, solid polyethylene can deleteriously adhere to interior reactor surfaces and reactor internals. If such adhesion is ongoing, the accumulated material can form solid lumps and interfere with reactor performance. Ultimately, if not remedied, this process of fouling may lead to a unit shutdown for cleaning.
Conventional systems have fed the ethylene through a nozzle without a length of pipe in the bottom of the reactor. The ethylene entered the reactor directly at the reactor wall, which led to fouling around this nozzle due to the very high concentration of ethylene and in the suspension. Fouling also occurred inside the nozzle itself. Due to low velocities of ethylene at the exit of the nozzle, catalyst-containing suspension would migrate into the nozzle and react with the ethylene to form polyethylene particles. To prevent total plugging of the nozzle, the nozzle would have to be cleaned frequently.
Therefore, a continuing need exists for ethylene slurry polymerization processes having improved performance through more efficient ethylene dissolution and mixing, resulting in reduced internal reactor fouling.